Several techniques are known in the art for growing crystals. The Czochralski process is the most widely used of these processes. In the Czochralski process, a heated crucible is provided for holding a liquid melt form of a charge material from which the crystal is to be grown. The melt is maintained at a temperature slightly above the solidification temperature of the charge material. A crystal seed is placed at one end of a cable or rod that enables the seed to be positioned into the melt material and then to be raised out of the melt material. The seed can be either a sample of the desired crystal material or any other material that has a higher melting temperature and the same crystal structure as the melt material in its solid form. When the seed is lowered into the melt material, it causes a local decrease in melt temperature, as is known to those skilled in the art, which results in a portion of the melt material crystallizing around and below the seed. Thereafter, the seed is slowly withdrawn from the melt and into a crystal growth chamber by a crystal puller system. As the seed is withdrawn, the portion of the newly formed crystal that remains within the melt essentially acts an extension of the seed and causes melt material to crystallize around and below it. This process continues as the crystal is withdrawn from the melt, resulting in crystal growth as the seed is continually raised away from the melt. While the crystal is being pulled it is also rotated about the axis along which it is being pulled so as to minimize undesirable temperature gradients in the crystal. When this process mode is effected, the crucible containing the melt also is rotated at a speed essentially the same speed as the crystal is being rotated so that force gradients in the crystal also are minimized.
The crystal is withdrawn from the growth chamber and into a transition chamber located above the crucible where the temperature is lower than that of the crucible. The crystal is finally drawn from the transition chamber into an elongated receiving chamber shaped to accommodate an extended length of crystal. Cooling of the crystal is achieved by passing a cooled heat exchange fluid, such a water, through the walls of the transition chamber which functions to remove heat from an inert gas surrounding the crystal and to remove radiant heat emitted from the crystal. It is current practice also to pass an inert gas from the receiving chamber into the transition chamber at a controlled temperature thereby to effect further cooling of the crystal as it passes from the transition chamber into the receiving chamber. The crystal growth process is time consuming, typically requiring about 72 hours, during which the seed crystal is slowly and continuously pulled away from the crucible containing the melt.
The crystal growth process is essentially a batch process. When the crystal fills the receiving chamber and production of the crystal is complete, the furnace is turned off and cooled down and the receiving chamber is decoupled either from the transition chamber or is decoupled, together with the transition chamber, from the growth chamber. The decoupled assembly, including the receiving chamber then is lifted away from the remaining portion of the crystal growth assembly and transported, usually by rotating it, to a station where the relatively heavy crystal can be safely recovered.
To restart the crystal growth process, the crucible is charged with the solid charge material for the crystal in an amount usually sufficient to complete the crystal growth, although some crystal growing systems have the capability to add solid charge material during the growth process by means of special feeder mechanisms. The receiving/transition chamber of the crystal growth apparatus is then coupled to the stationary portion of the crystal growth apparatus, a vacuum is established within the assembly, and the furnace is turned on to melt the charge material.
Since the receiving and transition chamber sections of the growing chamber are heavy, a conventional crystal growing furnace uses a primary hoist apparatus to lift and rotate the decoupled chambers upon completion of the crystal growth cycle and when initiating a new crystal growth cycle. In addition, some crystal growing systems use a feeder mechanism which contains a solid polycrystalline material used to charge the crucible at the beginning of a growth cycle. Since the charge material is heavy, a feed hoist is utilized to position the feeder for charging.
Further, a crystal growing furnace incorporates a process control operator's console in order to control process variables to assure production of satisfactory crystals. The process control console typically includes conventional elements such as a computer, programmable logical controllers, software and user interfaces. Among the process parameters regulated by the process control console are the rate and amount of energy supplied to the crucible, the rate of cooling (which is varied by controlling heat exchange fluid flow and, if present, inert gas flow within the transition chamber and/or receiving chamber), the rate of crystal rotation, the rate at which the crystal is withdrawn from the melt, rate of crucible rotation and rate of polycrystalline feed material to the crucible.
Conventional crystal growing systems are complicated and expensive to operate. The expense of running a large crystal growing operation where several furnaces are operating in parallel is considerable. First, conventional furnaces occupy large areas of expensive "clean room" floor space which could be used for other apparatus. Furthermore, the multiplicity of auxiliary apparatus required to service the furnaces requires a corresponding large maintenance effort to maintain the devices. It is also time consuming and labor intensive to set up each furnace and monitor each separate furnace station.
Accordingly, it would be desirable to provide a crystal growth system utilizing a plurality of crystal growing furnaces which is less expensive to operate than conventional systems.
In addition, it would be desirable to provide such a crystal growth system utilizing a plurality of crystal growing furnaces which is smaller and more compact than conventional crystal growing systems.